The invention relates to a method and to apparatus for measuring the diameter of a running elongate object such as an optical fiber, for example.
The diameter of an optical fiber needs to be monitored accurately during the manufacturing process, and the diameter must be measured while the fiber is running and without making contact therewith.
French Pat. No. 2,483,070 describes measuring the width or the diameter of a running object being manufactured, such as a strip, a rod, or a tube. The measurement is performed by means of two video cameras having optical axes which are substantially coplanar, mutually perpendicular, and oriented in such a manner as to intersect the running axis of the object perpendicularly. Each video camera is associated with two rows of photodetectors disposed symmetrically on either side of the optical axis of the camera and with means for scanning each of these rows of photodetectors from the end of the row furthest from the optical axis towards the end adjacent to the optical axis, thereby making it possible to determine the position on each row of the image of an edge of the object and thus to determine the apparent diameter and position of the object.
While it is running, the position of the object relative to its theoretical running axis may vary, due to vibration. The above-mentioned prior patent provides for using one of the video cameras to measure the displacement of the object along the optical axis of the other video camera, and to use this displacement measurement to calculate the appropriate correction for the measurement of the apparent diameter as performed by said other video camera.
However, such apparatus cannot measure with sufficient accuracy the diameter of a very fine object such as an optical fiber whose displacement relative to its theoretical running axis may be of the same order of magnitude as its diameter. The search for sufficient accuracy makes it necessary for each measuring channel to use a single row and single image-magnifying optics of reduced depth of focus, such that displacement of the optical fiber relative to its theoretical running axis gives rise to the formation of a blurred image on the rows of photodetectors, thereby preventing the diameter of the optical fiber and its position relative to its theoretical running axis from being determined. The aim of the invention is to solve this problem in a simple and effective manner capable of providing very much greater measuring accuracy than can be obtained by the means described in the above-mentioned prior patent.